The direction of a nacelle of a wind turbine generator is required to be controlled in the horizontal plane in accordance with the wind direction and speed. Such control is generally referred to as a yaw control, and a mechanism for carrying out the yaw control is generally referred to as a yaw control mechanism. A yaw control mechanism typically includes yaw drive devices for generating drive force to turn the nacelle in a desired direction and yaw brakes for securing the nacelle in the direction after the nacelle is directed in the desired direction. Most typically used as a yaw drive device is a drive mechanism which is configured to drive a pinion gear engaged to a gear provided on a tower with a yaw motor and a reducer. Such wind turbine generator is disclosed, for example, in United States Patent Application Publication No. US 2008/0131279 A1. On the other hand, most commonly used as the yaw brake is a hydraulic brake. The wind turbine generator using a hydraulic brake as the yaw brake is disclosed, for example, in Japanese Unexamined Patent Publication No. JP 2006-307653A.
Generally, a yaw drive device is designed to be able to generate a drive force sufficient to turn the nacelle in a sudden strong wind that occurs only once in 50 years (referred to as 50-year gust, which has a wind speed of 35 m/s or more, for example). This aims to allow a wind turbine rotor to face upwind even in a case where any strong wind occurs. It is desirable to face the wind turbine rotor upwind to reduce the wind load in the strong wind, since the wind turbine generator generally has a smallest wind load, when the wind turbine rotor is on the windward. The wind turbine rotor can be faced upwind to reduce the wind load in almost all cases, when the wind turbine generator is designed to generate drive force sufficient to turn the nacelle in the 50-year gust.
Here, according to an inventors' study, occurrence of such a sudden strong wind as the 50-year gust is very rare through a generally-demanded 20-year lifetime of a wind turbine, and the above-mentioned specification of the yaw control mechanism provides overquality for normal operation. Such specification of the yaw control mechanism undesirably leads to a high cost. Consequently, the inventors are considering a design in which the drive force of the yaw control mechanism is reduced, allowing the nacelle to be unable to be turned in a case where sudden wind external force has occurred.
The reduction in the drive force of the yaw drive device, however, may cause damage to the yaw motor when an instant unsteady wind occurred while the nacelle is turned to face the wind turbine rotor upwind. Specifically, when an instant unsteady wind such as the 50-year gust occurs during the turning of the nacelle, the nacelle may be pushed back instantaneously because of shortage of the turning torque. On this occasion, the nacelle is turned instantaneously at a high speed. Since the drive force of the yaw motor is transferred to the nacelle generally via the reducer, the yaw motor may be damaged because of the excessive rotation, when the nacelle is turned instantaneously at a high speed.